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Next-Generation Retinal Implant

Scientists plan to test an implanted chip with four times the resolution of the previous version in people blinded by retinal degeneration.
February 16, 2007

On Thursday, scientists at the University of Southern California (USC) announced their plans to test an improved retinal implant in blind patients. The new implant, which scientists hope will improve patients’ vision even more, has four times the resolution of the previous version. 

A tiny implant on the surface of the eye receives wireless signals from an external camera, which the patient wears on a pair of glasses. The implant transmits signals to an array of electrodes surgically implanted on the retina. The array delivers electrical signals to the nerve cells in the eye, mimicking the role of light-sensitive cells lost in degenerative retinal disease.

“My expectation, without really knowing what is going to happen, is that this will be useful for people in allowing them to find a lit doorway or the edge of an object when going into a room,” says James Weiland, a scientist at USC involved in the project.

People with retinal-degeneration diseases, such as retinitis pigmentosa and macular degeneration, lose their sight as the cells in the eye that normally sense light deteriorate. Retinal implants can take over for these lost cells, converting light into neural signals that are then interpreted by the brain. Simpler versions of these devices, developed by researchers at USC and other institutions, have already been tested in humans, giving patients rudimentary vision, such as the ability to detect light and to occasionally distinguish between simple objects. One patient, for example, wears the device to her grandson’s soccer games and reports that she perceives the sensation of the players’ movement as they run by, says Weiland.

The device, developed by Mark Humayun and colleagues at USC, consists of a tiny chip dotted with hair-thin electrodes. When implanted in the retina, the electrodes transmit electrical signals from the chip to neural cells in the eye, which then send the message to the brain. A wireless camera mounted on glasses and a video processing unit worn on the belt capture and process visual information from the wearer’s surroundings and wirelessly transmit those signals to the chip.

The new version of the implant, which the researchers have been working on for the past eight years, has nearly quadrupled the number of electrodes–from 16 to 60–and is about half the size of the previous model. The researchers recently received permission from the Food and Drug Administration to start human tests, which they plan to begin in the next few months.

Once the device is implanted, researchers will need to do extensive tests to figure out how to optimize it. “A camera gets at least tens of thousands of pixel information, and we need to transmit that to just 60 stimulating channels,” says Weiland. “We have to figure out what is the most important information to keep.”

Increasing implant resolution by a factor of four is significant, says E. J. Chichilnisky, a neuroscientist at the Salk Institute for Biological Studies, in La Jolla, CA. But compared with the human eye, the resolution is still very limited. “Imagine a camera with 60 pixels,” Chichilnisky says. “You can’t really see a face in an eight-by-eight image, or even a word. In the long run, we’ll need hundreds or thousands of electrodes to get something interesting. So there is a lot more to be done.” Both Chichilnisky and the USC researchers are working with Second Sight Medical Products, the company based in Sylmar, CA, that is manufacturing the devices, on the next version of the implant. The third-generation device will have 500 electrodes, boosting resolution by a factor of almost 10.

But increasing the number of electrodes won’t be the only hurdle in developing implants that can give blind people truly useful vision. Scientists also need to figure out how to electrically stimulate the retina in a way that the brain can interpret with high spatial resolution, says Joseph Rizzo, an ophthalmologist at the Massachusetts Eye and Ear Infirmary and codirector of the Boston Retinal Implant Project. A ray of light, for example, stimulates retinal cells in a more precise and refined way than does the electric current coming from an electrode. “It doesn’t matter if you have 10 or 1,000 electrodes,” he says. “If you don’t know how to use them, it doesn’t matter.”

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